Three-wire attachment plug receptacle



Marqh 16, 1954 w SHENTON 2,672,593

THREE-WIRE ATTACHMENT PLUG RECEPTACLE Filed June 15, 1952 Q QQSfiHOrIQQ /S Patented Mar. 16, 1954 UNITED STATES ATENT OFFICE THREE-WIRE ATTACHMENT PLUG RECEPTACLE Application June 13, 1952, Serial No. 293,266

6 Claims.

This invention relates to attachment plug receptacles and more particularly to attachment plug receptacles for use with plugs having three prongs, such, for example, as are used in threewire circuits.

Heretofore attachment plug receptacles for use with wall plates or outlet box covers have been made mainly in the form of a single or duplex outlet. The single outlet form usually has a round front face portion adapted to fit within or extend through a round hole in the wall plate. The duplex form customarily has two face portions raised above the front surface of the receptacle body, each shaped as a circle flattened at opposite poles for known reasons,

In designing a new three-wire form of duplex receptacle, it was a practical necessity that its face portions be dimensioned and shaped to fit within the holes of a standard duplex wall plate. Otherwise, a new design and new tools would also be necessary for a special wall plate which would add to the cost and the stock of parts a jobber need keep on his shelf.

The need to place all the openings for the three prongs of the plug cap within the limited area of a face portion of the receptacle body imposes serious limitations on design. Among those limitations a few may be mentioned. Firstly, it is necessary (if the receptacle is also to be capable of receiving standard two-prong attachment plug caps) for the slots through which the line-wire prongs are to pass to be of the same size and arranged the same distance apart as in conventional two-wire receptacles. Secondly, the aperture for the grounded prong must be properly spaced from the aforesaid slots. Thirdly, it is desirable for reasons of manufacturing economy that the prong receiving contacts within the receptacle body be formed so as to accept prongs arranged either in parallel or in tandem, both of which arrangements are common. Therefore a relatively large portion of the interior of the receptacle is occupied by those contacts, leaving only a small amount of room for the grounded contact. Such limitations indicated the, need to place the opening for the grounded prong close to one edge of the face of the outlet. Such placement gave rise to a new difiiculty when a grounded prong of greater length and girth was used, as is desirable in certain instances. Users would jerk the plug from the outlet applying lateral pressure by pulling on the wire or would manually pull or otherwise press the plug sidewise. The

outlet would break away or chip oil at the edge adjacent the ground-prong aperture.

Therefore it is an object of this inventionto provide an improved three-wire electric outlet or attachment plug receptacle usable with standard 'face plates which will accept three-prong plugs with a large grounded prong which will withstand strenuous usage and abuse, particularly sidewise pulling and laterally applied stresses.

Other objects and advantages will appear as the invention is described in connection with the accompanying drawings.

The invention is illustrated as applied to a duplex attachment plug receptacle adapted to re ceive a plug having parallel prongs connected to the line wires and a grounded prong. It will be apparent however that the invention is applicable to a single receptacle and also, in either single or multiple form, to plugs in which the wire-connested prongs are arranged in tandem or otherwise.

In the drawings:

Figure 1 is a plan view of a duplex receptacle in which the invention is applied;

Figure 2 is a bottom view of the cover portion of the receptacle illustrated in Fig. 1;

Figure 3 is a side elevation view partly in section along line 3-3 of Fig. 1 showing the plug being removed;

Figure 4 is a view similar to Fig. 3 showing for comparison a receptacle without the invention applied thereto and illustrating the likelihood of breakage in the three-wire type of receptacle;

Figure 5 is an enlarged detail view in sectional elevation similar to Fig. 3;

Figure 6 is an enlarged detail view showing in sectional elevation a portion of the structure of Fig. 4.

Referring to the drawings, a molded, plastic, insulating body of generally conventional exterior shape has a base portion it which is hollowed out and recessed to receive and hold combined contact and terminal members. A cover member 20 also molded of insulating material forms the top of the insulating body and fits on the base portion ID to enclose the contact and terminal members. The cover has two raised faces 2| of size and shape to fit within the openings of a standard wall plate for attachment plug receptacles.

The contact and terminal members I2 are stamped from sheet stock in usual form and may, if desired, be formed to accept either parallel or tandem plug blades. The grounded contacts M are also stamped from sheet stock into usual substantially U-shape and riveted through the receptacle bottom to the mounting strap l5.

The cover 20 i provided with pairs of parallel spaced rectangular slots 22, 24 to receive the parallel 'blades or prongs of a conventional twowire attachment plug cap or the similar parallel blades or prongs of a three-wire attachment plug cap 30. To receive the third or grounded prong 32 of the three-wire attachment .plug cap, a U-shaped aperture 26 is formedinthe'c'over and located preferably in the longitudinal axis thereof adjacent to but spaced from the parallel slots. Each set of slots and their ground-prong aperture lie within the area of one-outletfac'e H.

The above-mentioned three-prong plug 30 is a standard form. Two of the prongs 34, 36 are the blade-type and are similar to standard twowire plug prongs. The third or ground prong 32 may be in the form of a solid or hollow pin,

but preferably isastrip of metal folded lengthwise by stamping, into U-shaped cross-section. Those .forms of grounded prong prevent entirely insertion or forcing of the'prongs into the Wron receptacle slots. The desire to employ such form 1 of plug further handicaps freedom of design within the already limited space.

Because of the space limitations, it is necessary to placethe aperture for'the grounded prong "32o; the attachment plug cap close to the edge 'of the protruding face 21 of the receptacle. I

have found as a result of meeting this requirement that there is a danger of the relatively, and necessarily, thin wall l of the projecting portion breaking away as at 13 when the sidewise or lateral thrust .is applied to the plug as,

-for example, when the plug is pulled out by a sidew'ise jerkon the electric cord which is always attached to it. Thi is due (see Fig. 6) to the leverage applied at the upper edge or point I! of "the prom; aperture where the prong 32 presses as force is applied in direction of the arrow 'X. The prong, acting as a lever, has'afulcrum point 49 "atth'e' inner lower corner or edge on the opposite side of thepiong 'ap'ert'u're where it is off- :set and'u'r'i'dene'd to providespace for movement of the fingers of the grounded receptacle contacts ll. Obviously as the prong is withdrawn, the lever -arin,'zb'etween theiulcrumpoint andthe point where the pressure is applied, increases 'hecause-oi" the plug movement farther and fartherfrom the receptacle. But, the shorter force :arm, .between points I1 and I9 remains constant.

Hence with thesame lateral pull applied to the ,plug, the force acting to break the wall "5 increases as the prong 32 is withdrawn. Whether the plug is fully inserted 'or partly withdrawn,

"a destructive force isapplied. That force is mul- "tiplied'greatly 'whenthe plug is jerked, but itis nevertheless present when a steady, strong, ob-

ilque pull i's-ex'ert'ed;

To avoid the exertion 615 such destructive pressure on the wall it, I bevel the inner arcu'ate wall 21 of ground-prong aperture 26 inwardly-of :the cover-away from the outer margin 29 of the :aperture. -In that way, the fulcrum point I9 is removed. Removal of the f-ulcrumpoint allows are partly withdrawn, the absence of the fulcrum point allows even greater tilting, further reducing the lateral component. At this stage, such reduction is important because less of the stress on the prongs is taken up by the receptacle contacts.

.From the :foregoing',.it will be clear that inability to strengthen the thin receptacle wall 16 by thickening or otherwise, demands that the stress be relieved, removed or transferred. Hence .theprong mustbe able to tilt. By reforming the interior of the ground plug aperture to provide an inclined inner wall opposit the thin wall,

removal of the :fulcrum point is accomplished -rnentplug cap and a third openingto-receive'a grounded prong of a, three-wire attachmentplug cap, said third opening being near one margin of said area and having a relatively-thin frangible wall-between it andsaid margin, the wall of said third opening opposite said thin wall being inwardly offset and widened to provide space for movement of the finger of a receptacle contact,-and said wall also being undercut inwardly between said offset and said front face of the receptacle thus to enlarge inwardly from the face of the receptacle, and thereby to permit tilting movement of the grounded prong when in partly withdrawn position.

2. A receptacle as claimed in claim 1 wherein thethird opening is U-shaped peripherally with the straight side a'dja'centth'e thin wall, and the wall opposite the thin wal1 is bevelled inwardly progressively to said offset portion.

3. Th construction of an attachment plug receptacle body of the three wiretype in which the slots "for the prongs "or a three-prong plug must lie within the face area of the portion of the receptacle which fits with-in the opening of a :standard receptacle wall :plate, comprising a receptacle body having two slots located and-dimensioned to receive the prongs of a standard two-prong plug and a third slot receiving the grounded prong of athree-wire plug, said third .slotbe-ing located within an area whichincludes said first two-slots and which is of a size less than th opening in a standard receptacle wall plate, a thin frangiblew'all between said third :slotand-the margin of sai'd area, the interior of "said third slot increasing in area inwardly by "the'wall opposite said thin wall beingbevelled inwardly to enlarge said third slot from front toward-the rear of the receptacle whereby tiltin'g-oi said grounded prong is made possible to avoid breaking said thin wall when lateral pres- .sure isapplied to said plug.

5 6 thereon, said two slots and said recessed portion D of the third slot being in said cover portion, References clted in the me Of thls Patent thglilebydfacgitatiflig the molding of said cover UNITED STATES PATENTS w1 sa1 un ercu por Ion.

6. The construction as claimed in claim 1 5 i g gg z fg g M 1 wherein the receptacle is of molded insulation 1927245 Russ an e 1933 having a base portion and. a cover portion fitting 2051856 Hubbell 1936 thereon, said pair of openings and said under- 2214065 z' i 1940 cut of th third opening being in said cover porp tion, thereby facilitating the molding of said cover 10 with said undercut portion.

THOMAS W. SHENTON. 

